As a format of optical disks for recording/reproducing information, DVD-R/RW is used. The first feature of the DVD-R/RW format is that address information is formed on a land in a gap portion between guide grooves (referred also as “grooves”) on an optical disk to increase the compatibility with a DVD-ROM format. This address is referred to as a “land pre-pit address” or a “LPP address”.
The second feature is that the guide grooves undulate in a radial direction in a certain cycle to form a wobble. A wobble signal obtained based on the wobble is used as a reference signal for generating a clock for recording and reproducing information.
This format will be described with reference to FIG. 6. FIG. 6 is a view schematically showing a shape of the grooves on an optical disk. A track as a region where information is to be recorded as a mark is formed of a groove 101. A land 102 is formed between the grooves 101. A recording mark 103 is formed in the groove 101, and a land pre-pit 104 is formed on the land 102. As shown in the figure, the grooves 101 undulate in a lateral direction, i.e., a radial direction of the optical disk, to form a wobble. In a DVD-R disk or a DVD-RW disk, a cycle of the wobble is 186 times that of a recording clock. The land pre-pit 104 is encoded with address information and is used to detect the exact position on the disk.
FIG. 6 shows a tracking detector 105. The tracking detector 105 is provided as a part of the elements of an optical head (not shown), and detects laser light reflected by the optical disk and generates a tracking servo signal for allowing the laser light to follow the groove 101. FIG. 6 shows a state in which the tracking detector 105 is divided into two tracking detectors A and B by a dividing line 106 extending in a direction of the tracks (track direction). In other words, the tracking detectors A and B are arranged side by side in the radial direction of the optical disk. Each of the tracking detectors A and B detects an amount of laser light reflected by the optical disk.
The detection signal from the tracking detector 105 is used to detect not only the tracking servo signal but also other signals. LPP address information is detected based on a differential signal indicating a difference between a plurality of light amount signals from the tracking detector 105. Recorded/reproduced information is detected based on a sum signal indicating a sum of the plurality of light amount signals. As in the case of detecting the address information, a wobble signal also is detected based on the differential signal indicating a difference between the plurality of light amount signals. FIG. 6 shows a state in which, in order to detect the wobble and the LPP address, the outputs from the detectors A and B are subjected to predetermined processing, then input to a wobble detection differential amplifier 107 and a LPP detection differential amplifier 108, and converted into differential signals indicating a difference therebetween.
As shown in FIG. 6, in a conventional optical disk apparatus, the wobble detection differential amplifier 107 and the LPP detection differential amplifier 108 are provided as separate elements (see, for example, JP 2002-216363 A). The reason for this is as follows.
In the DVD-R/RW format, the guide grooves for recording information are formed with the wobble as described above. Locally, the position of the optical head is displaced from the center of a track at a certain frequency with respect to the track. Consequently, in recording information, an imbalance is created between amounts of light incident on the two tracking detectors A and B, and a recording signal is mixed in an address signal.
Waveforms (a) to (d) in FIG. 7 are waveforms at respective portions in recording by the optical disk apparatus. The waveforms (a) and (b) are waveforms of the output signals from the tracking detectors A and B, respectively. Since the tracking detectors A and B detect reflected light of an identical light spot, when one of the detectors outputs a higher amount of light, the other outputs a lower amount of light. Further, both the tracking detectors A and B detect the land pre-pit 104, although their detection levels are different.
In FIG. 7, a recording signal component S indicated by a solid line, a wobble signal component W indicated by a dashed envelope, and LPP signals La and Lb are shown in an emphatic manner. The recording signal component S in the waveform (a) is in phase with that in the waveform (b). The wobble signal component W in the waveform (a) is in opposite phase to that in the waveform (b). The LPP signals La and Lb are located at right and left peaks, respectively, of the envelope indicating the wobble signal component W. At the left peak, the LPP signal La is shown, which corresponds to the case where a recording signal is irradiated on the LPP (irradiation of a peak power). At the right peak, the LPP signal Lb is shown, which corresponds to the case where the LPP is in a valley of irradiation of recording signals (irradiation of a bottom power). In the waveform (a), the output signal is higher than the envelope at a position of the LPP signal La on the left side. On the other hand, in the waveform (b), the output signal is lower than the envelope at a position of the LPP signal La on the left side. At the right peak, a component of the LPP signal Lb based on an amount of light by the bottom power is detected in opposite phase on the lower envelope.
The waveforms (c) and (d) are waveforms of difference signals obtained by subtracting the signal of the waveform (b) from the signal of the waveform (a). The waveform (c) shows the output from the wobble detection differential amplifier 107 in FIG. 6, corresponding to the case where an average value of the recording signal component S in the waveform (a) is equal to that in the waveform (b). The waveform (d) shows the output from the LPP detection differential amplifier 108, corresponding to the case where the recording signal component S in the vicinity of the peaks of the wobble signal where the LPPs are present in the waveform (a) is equal to that in the waveform (b). In either case, an imbalance amount of the recording signal component S varies in a cycle of the wobble signal in principle.
In the waveform (c), a residual component of the recording signal component S (hereinafter, referred to as a “residual signal component”) is minimum in the vicinity of an average value of the difference signal, is maximum in the vicinity of the peaks of the wobble signal where the LPP signals La and Lb are present, and is maximum in a negative direction in the vicinity of an opposite phase of the wobble signal. In the waveform (d), the residual signal component is minimum in the vicinity of the peaks of the wobble signal where the LPP signals La and Lb are present, is maximum in a negative direction in the vicinity of an opposite phase thereto, and is substantially equal to the negative peak value in the waveform (c) in the vicinity of the average value.
In subtracting the signal of the waveform (b) from the signal of the waveform (a), when an appropriate balance is set in level between these signals, the LPP signal La on the left side can be detected in both the waveforms (c) and (d). On the other hand, the LPP signal Lb on the right side in the waveform (c) is buried in the residual signal component and cannot be detected.
As described above, when the differential balance is adjusted so that average amounts of light incident on the two tracking detectors A and B are equal, the wobble signal can be detected accurately by using the center of the amplitude of the differential output as shown in the waveform (c) in FIG. 7. However, in the waveform (c), it is sometimes difficult to detect the land pre-pit of the LPP signal Lb. The reason for this is as follows. The land pre-pit is recorded at a position where the optical head is off-track relatively for wobble processing. Thus, mixing of the recording signal becomes maximum at the recording position, and in the case of irradiation of the bottom power, the LPP signal is buried in the residual signal component. To avoid this, the amplitude of the recording signal is detected at the recording position of the land pre-pit, and the differential balance outputs from the tracking detectors A and B are adjusted to be equal at that position. Consequently, as shown in the waveform (d) in FIG. 7, mixing of the recording signal component can be minimized at the recording position of the land pre-pit. As a result, the detection rate of the land pre-pit can be increased.
However, in the differential balance outputs adjusted as in the waveform (d) in FIG. 7, jitter is increased in a binarized wobble signal, and the above processing is not suitable for the binarized wobble signal. In general, the binarization of the wobble signal is performed by a method of binarizing the wobble signal that has passed through a band-pass filter at a certain slice level or by a duty feedback slice method in which the binarized signal has a duty ratio of 50%. However, by using either method, when the wobble signal output from a band-pass filter is binarized in the vicinity of the center of the amplitude, jitter is increased in the binarized wobble signal. The reason for this is as follows. When the differential balance is output so as to increase the detection rate of the land pre-pit, mixing of the recording signal component is minimized at the recording position of the land pre-pit, but is increased on the contrary in the vicinity of the slice level of the wobble signal.
With the foregoing in mind, it is found that the optimum adjustment point is different between the adjustment of the differential balance for detecting the wobble and that for detecting the land pre-pit. To cope with this, in the conventional example described in JP 2002-216363 A, a wobble detection balance adjustment circuit for adjusting the outputs from the two tracking detectors A and B for detecting the wobble and a LPP detection balance adjustment circuit for adjusting the outputs for detecting the land pre-pit are provided as separate elements. Accordingly, in order to output the differential signals between the outputs from the two balance adjustment circuits, the two differential amplifiers, i.e., the wobble detection differential amplifier 107 and the LPP detection differential amplifier 108 are provided as separate elements. With this configuration, both the wobble signal and the land pre-pit can be detected accurately.
As described above, the conventional optical disk apparatus requires the two balance adjustment circuits and differential amplifier circuits for detecting the wobble and the LPP, respectively. When an analog circuit is increased in scale, it consumes a larger amount of power, and the operation becomes unstable due to a circuit offset and temperature characteristics. To cope with this, it is necessary to provide a countermeasure circuit, which, however, degrades an S/N and is unfavorable for delicate detection of a signal having various patterns. However, when such processing by an analog circuit is to be performed digitally, two high-speed and high-accuracy analog-to-digital converters are required. A high-speed and high-accuracy analog-to-digital converter has a large circuit scale and consumes a large amount of power as compared with other analog circuits or a digital arithmetic circuit even at the present time when circuits have become miniaturized and faster. Therefore, it is desirable to minimize the use of a high-speed and high-accuracy AD converter.